In recent years, a capacitor of a greater capacity and a higher output has drawn attention from the market as a product to be used in a power supply which drives a motorcar. This capacitor is now in a development stage. This kind of capacitor charges or discharges electricity, and its structure is known as an electric storage device disclosed in patent document 1.
This conventional electric storage device includes a wound element formed of an anodic electrode unit, a cathodic electrode unit, and a separator for insulating both the electrode units from each other. A positive collector is disposed on a first axial end and a negative collector is disposed on a second axial end of the wound element, and the positive collector is connected to the anodic electrode unit and the negative collector is connected to the cathodic electrode unit. These members are accommodated in a cylindrical housing having a bottom plate, and electrolyte is filled in the cylindrical housing. The opening of the housing is covered air-tightly with a lid.
The positive and negative collectors are formed of a belt-like electrode foil, on which both sides polarization electrodes are formed. The axial end of the electrode foil on the anode side protrudes to a first side, and another axial end on the cathode side protrudes to a second side, and both of the projected ends have no polarization electrodes. A region having the polarization electrodes and a region having no polarization electrode are formed on the belt-like electrode foil such that they have a given width along the longitudinal direction. The region having no polarization electrodes of the foregoing wound element is bent in part and brought into contact with the positive collector and the negative collector, and then connected to the collectors by, e.g. welding.
FIG. 20A shows a sectional view of a conventional cylindrical capacitor cut along the axial direction. FIGS. 20B, 20C, 20E, and 20F show sectional views of the wound element of the conventional cylindrical capacitor. FIG. 20D shows a fellow to the electrodes of the belt-like electrode unit of the wound element. FIGS. 20C and 20F show enlarged views around collectors 2023 and 2025 of the electrode unit of the wound element shown in FIGS. 20B and 20E. FIGS. 20B and 20C show the status where the wound element has been just rolled up, and FIGS. 20E and 20F show the status where collectors 2023 and 2025 are bent.
As FIG. 20F shows, collectors 2023 and 2025 placed in first region 2021 and second region 2022 of the electrode unit are bent to form collectors 2023 and 2025. As shown in FIG. 20A, lid 2003 working as an anodic member is brought into contact with collector 2023, and cylindrical housing 2002 with a bottom plate working as a cathodic member is brought into contact with collectors 2025, then they are connected by, e.g. welding, thereby forming a first pole and a second pole. The electrolyte is subsequently poured into housing 2002 through hole 2011, which is then closed with stopper 2012. The capacitor is thus completed.
The cylindrical capacitor discussed above needs to reduce its resistance as much as possible in order to meet the requirements of internal resistance depending on the applications. The internal resistance exists in each element of the capacitor, namely the lid working as an anodic member, the cylindrical housing with a bottom plate working as a cathodic member, the electrolyte, the wound element, joint sections between the lid, cylindrical housing and the wound element. For instance, in the case of the conventional structure disclosed in the patent document 1, one of effective means for lowering the resistance is to reduce the width of the region, having no polarization electrodes, in the collector of the electrode unit, so that a plane area of the collector can be reduced and the resistance of the wound element is thus lowered.
However, the width reduction in the wound element discussed above forces the collectors to bend in order to obtain the connecting area between the lid, the cylindrical housing, and the wound element, so that the distance between the separator and the lid, cylindrical housing, wound element is shortened at both the ends of the wound element. The welding for connecting the collector to the lid and cylindrical housing thus sometimes burns the end of the separator, and the insulation between the anodic electrode unit and the cathodic electrode unit becomes in failure.
The cylindrical capacitor discussed above needs a positive insulation between the anode and the cathode, and on top of that, if an apparatus to which the capacitor is mounted is vibrated, the wound element is required not to come off from the lid or the cylindrical housing. To meet this requirement, the lid or the housing is provided with a protrusion at its center, and they are inserted into a winding shaft of the wound element to be tightly assembled. The center of the wound element thus agrees with that of the cylindrical housing so that a given clearance between the wound element and the cylindrical housing is preferably prepared for fixing the wound element to the housing. In the conventional cylindrical capacitor disclosed in patent document 1, collectors 2023 and 2025 of the electrode units are bent toward winding shaft 2020 of wound element 2017, i.e. toward the core of the wound element as shown in FIGS. 20A, 20E and 20F, so that an open end of winding shaft 2020 is closed by the collectors of the fore-going electrode unit. This fact reveals that the insertion of the protrusion prepared at the center of lid 2003 or cylindrical housing 2002 into winding shaft 2004 of wound element 2001 sometimes breaks collectors 2023 and 2025 in parts. Thus the protrusion prepared at the center of lid 2003 or housing 2002 is not recommended, so that it is not possible for fixing wound element 2001 to housing 2002 with the center of wound element 2001 agreeing with the center of cylindrical housing 2002.
The capacitor disclosed in patent document 2 has an advantage with a view of lowering the resistance of the wound element. FIGS. 21A-21D show the structure of this capacitor. FIG. 21A show an electrode unit and a separator before they are assembled into the wound element. FIG. 21B shows a status where collectors at both the ends of the wound element are welded. FIG. 21C shows a plan view of the electrode unit of the capacitor disclosed in patent document 2, and FIG. 21D shows a plan view of the electrode unit of the capacitor disclosed in patent document 1.
The capacitor disclosed in patent document 2 includes the following elements:                anode-side electrode unit 2101 formed of collector 2103 having flaring section 2104 of which width increases from a first end of a long side to a second end thereof and polarization electrode 2105 formed on collector 2103 except in the region where flaring section 2104 exists; and        cathode-side electrode unit 2102 formed of collector 2106 having flaring section 2107 of which width increases from a first end of another long side to a second end thereof, and polarization electrode 2108 formed on collector 2106 except in the region where flaring section 2107 exists.These elements are rolled together with separators 2109, 2110 placed therebetween using the first end of the long side as the center of the roll. At both the ends of wound element 2111 thus rolled up, anode-side flaring section 2104 and cathode-side flaring section 2107 are bundled and welded together respectively to form collectors 2112 and 2113, which are then connected to terminals 2114, 2115 respectively. The width of anode-side flaring section 2104 and the width of the cathode-side flaring section 2107 flare opposite to each other with respect to the width direction of anode-side electrode unit 2102 respectively.        
As show in FIG. 21C, electrode unit 2116 disclosed in patent document 2 measures M on its long side, and measures λ on the width of polarization electrode 2118, and the first end of the longitudinal side measures H1+λ, and the second end thereof measures H2+λ. The plane area S0c of collector 2117 is thus calculated with the following equation (1):
                              S          ⁢                                          ⁢          0          ⁢          c                =                              1            2                    ⁢                      M            ⁡                          (                                                H                  ⁢                                                                          ⁢                  1                                +                                  H                  ⁢                                                                          ⁢                  2                                +                                  2                  ⁢                  λ                                            )                                                          (        1        )            where H1<H2 is established.
On the other hand, according to the structure shown in patent document 1, electrode unit 2119 measures M on its long side and polarization electrode 2121 measures λ on its width as shown in FIG. 21D. The first and the second ends of the long side measure H2+λ. Plane area S0d of collector 2120 thus figures out M(H2+λ). The difference between S0c and S0d is thus calculated with equation (2).
                                          S            ⁢                                                  ⁢            0            ⁢            c                    -                      S            ⁢                                                  ⁢            0            ⁢            d                          =                              1            2                    ⁢                      M            ⁡                          (                                                H                  ⁢                                                                          ⁢                  1                                -                                  H                  ⁢                                                                          ⁢                  2                                            )                                                          (        2        )            Since H1<H2 is established as discussed above, S0c<S0d is established. As a result, the collector of the conventional capacitor disclosed in patent document 2 has a smaller plane area than that of the capacitor disclosed in patent document 1. It will be detailed later but as shown in FIG. 6, the collector has a smaller resistance with its smaller plane area. Therefore, the electrode unit of the wound element of the conventional capacitor disclosed in patent document 2 has a smaller resistance than the electrode unit that of conventional capacitor disclosed in patent document 1. As shown in FIG. 21B, the structure of the capacitor disclosed in patent document 2 allows preventing the edge of electrode unit of the collector from protruding at the edge of the wound element, and also allows a positive connection between the collector and the external terminals of an electric storage device.
According to the structure of the conventional capacitor disclosed in patent document 2; however, the collector of the electrode unit closes the winding shaft of the wound element as shown in FIG. 21B. This is also observed in the conventional capacitor disclosed in patent document 1. The winding shaft closed by the collector prohibits the shaft from being fixed to the housing by using the projection prepared at the center of the lid or the cylindrical housing. As a result, it is difficult for a capacitor to insulate positively between the anode and the cathode, or to obtain the reliability in the vibrating circumstances.    Patent document 1: Unexamined Japanese Patent Application Publication No. H10-294102    Patent document 2: Unexamined Japanese Patent Application Publication No. 2002-56886